You get the buff for the time it is active - 20 sec.

As far as the proc goes, you have to recast the ability after you use the proc in order to do the 4 attack-proc thing again. At least that is the way it works for me.

Isn't mer resolve single target though? How do you do your main aoe

I personally find that merciless resolve alone makes dual wielding a less attractive option as a magblade. It's true that higher spell damage can be reached with dual wield, however with Assassin's Will procs as high as 60k, I find that to be an unequal trade-off. The shorter the fight, the less group cohesion and group buffing (like powerful assault, SPC, war horn, etc.) the more viable dual wield becomes, because you are more reliant on your own buffs than on others. Running in a group that knows what they are doing and constantly has group buffs active, forcing yourself into melee to proc AW is inefficient.

I believe that MR should be on your single-target bar. In PvE, assuming you weave properly throughout the entire fight, bar-swapping every 2-3 seconds just to cast AW is highly inefficient. It makes more sense for your short-duration buffs to be on the bar that is most readily accessible (provided you have that option, which you do in this case). Once AW is cast, MR needs to be re-cast or you will not see another AW proc.

I do things a little differently than the other cookie-cutter builds around these parts, and I stick with destro staves on both bars. Assuming you're using the vMA destro, it turns out to be better SD anyway, because with DW you aren't getting the full benefit of your off-hand weapon, so the extra 189 SD from the staff outweighs the ~52 SD you get from your off-hand sword. That being said, DW produces higher SD values in pretty much every other scenario, with vMA destro being the catalyst.

The only benefit you have to putting MR on your aoe/off bar is the crit chance you get for having it slotted, and that isn't so substantial that it outweighs the benefits of having it on your primary bar. It's also substantially easier to empower your AW procs with something like Inner Light if it's already in your primary bar rotation. In addition, AW counts as a light attack as it pertains to Kena, so if you light attack > AW, you will proc kena without a second light attack, meaning you can continue your rotation with seamless kena procs.

Usually on the off-bar you have mostly aoe and utility. I have Inner Light on both bars, so the other skills are: Sap, Twisting Path, Elemental Blockade, Siphoning Attacks. Primary bar is Crippling Grasp, Swallow Soul, Impale, Merciless Resolve.

Also a tip I think might be helpful, if not for the OP, then for anyone else experimenting with a magblade: Soul Harvest > empower > Assassin's Will results in extremely high numbers. My highest AW proc crit for 62k this way, and I can fit this into my normal rotation. Usually SH is on my aoe bar (so that aoe burning recharges ult super fast) so it's relatively easy to reapply my AoE DoTs, cast Soul Harvest, bar swap, and do my normal rotation, allowing for almost the full 6 seconds of extra damage. Even if AW isn't proc'd yet at this point, it's usually quite easy to proc it before the damage buff wears off, and magelight can be used at any time. It's also good practice to ensure proper light-attack weaving even during aoe phases. Doing so will enable you to switch at any time to throw out an AW proc (which is very useful for situations like vDSA, where aoe on adds is important, but you have to kill a sacrifice or something. There are other applications, this was simply my first thought).